频率表达：always/usually/sometimes — 英语阅读题

阅读短文，回答问题。 My name is Nina. This is my free time. I always read books in the evening. On Saturday, I usually go skating with my friends. But sometimes I stay at home and sing. My brother Sam is different. He never reads books. He always plays football after school, and sometimes he paints pictures with me. Question: What does Nina sometimes do on Saturday?